Colour Blindness Inheritance Explained
Red-green colour blindness is an X-linked recessive trait. This means the gene responsible is on the X chromosome. Females (XX) must inherit two copies of the recessive allele (XcXc) to be affected, while males (XY) only need one (XcY).
Parental Genotype Determination
We determine the genotypes based on the information given:
- Father: He is colour-blind. As it's X-linked, his genotype is definitively XcY.
- Mother: She is phenotypically normal but has had a colour-blind son (XcY). This son received his Xc chromosome from his mother. Since she is normal, she must also have a normal allele (XC). Thus, her genotype is XCXc (she is a carrier).
Probability Calculation via Genetic Cross
The genetic cross is between the mother (XCXc) and the father (XcY). We can use a Punnett square to visualize the possible offspring genotypes:
| Mother's Gametes | |||
| XC | Xc | ||
| Father's Gametes | Xc | XCXc (Daughter, normal phenotype) | XcXc (Daughter, colour-blind phenotype) |
| Y | XCY (Son, normal phenotype) | XcY (Son, colour-blind phenotype) | |
We need the probability of a colour-blind daughter. A colour-blind daughter has the genotype XcXc.
- Looking at the possible daughter genotypes (XCXc and XcXc), one out of the two possibilities results in a colour-blind daughter.
- The probability of inheriting Xc from the mother is 1/2.
- The probability of inheriting Xc from the father is 1.
- The probability of a daughter being XcXc is $(1/2) \times 1 = 1/2$.
Final Probability Result
The probability of having a colour-blind daughter is 50%.
Important Questions from Mendelian Inheritance
The allele associated with albinism in humans is recessive ($c$). The probability that an albino male ($cc$) and a carrier female ($Cc$) will have an offspring with normal skin pigmentation is _________.
(Round off to one decimal place)
- The blood group of the mother is A⁺ and that of the father is AB⁺. Which of the following statements is/are correct?
- Fabry disease in humans is a X-linked disease. The probability (in percentage) for a phenotypically normal father and a carrier mother to have a son with Fabry disease is ________.
A pedigree of an inheritable disease is shown below.
- In tomato plant, red (R) is dominant over yellow (r) for fruit color and purple (P) is dominant over green (p) for stem color. Fruit color and stem color assort independently. The number of progeny plants of different fruit/stem colors obtained from a mating are as follows:
Red fruit, purple stem - 145
Red fruit, green stem – 184
Yellow fruit, purple stem - 66
Yellow fruit, green stem - 47
What are the genotypes of the parent plants in this mating?
